1. Field of the Invention
This invention relates to an image processing apparatus and an image processing method for converting frequency characteristics of color image data in full-color printing equipment and related equipment such as printers, video printers, and scanners, image generating equipment for creating computer graphic images, or color display equipment, in accordance with the particular equipment used or conditions of use.
2. Description of the Related Art
Image data input to color image display equipment and the like generally consist of red (R), green (G), and blue (B) color data. The color data are generated by a camera or other image generating apparatus, but are subject to the effects of various types of noise in transmission, so the color data input to the image display equipment do not necessarily match the original color data. Processing such as removal of noise components is therefore performed at the image display equipment so that the original image can be reproduced.
Patent Document 1, listed below, gives an example of an image processing apparatus that smoothes an image. From input image data, the image processing apparatus described in this patent document distinguishes between light and dark parts of the image and carries out the smoothing process only on light parts that are adjacent to dark parts, to prevent an apparent thinning of lines when dark characters are displayed on a light background.
In addition to this type of image processing, color conversion processing is carried out to correct color data according to device characteristics so that desired color reproduction is obtained. Desired color reproduction means reproduction of colors that people will like, giving consideration to human perceptual characteristics and color memory, which is not necessarily the same thing as reproducing colors faithfully. Human color memory has a tendency to remember colors such as the color of the sky and the color of a green lawn as being more vivid, that is, brighter and more saturated, than they actually are. Processing is therefore performed to increase the brightness and saturation of particular color components such as these. Even when colors are reproduced faithfully, it is not unusual to perform processing to increase the brightness and saturation.
A feature of the color conversion apparatus described in Patent Document 2 below is that it performs color conversion processing by a matrix calculation on terms valid for six hue components in a color image: red, green, blue, yellow, magenta, and cyan. By appropriate selection of the matrix coefficients for the terms valid for these six hue components, it is possible to adjust the red, green, blue, yellow, magenta, and cyan hue components independently.    Patent Document 1: Japanese Patent Application Publication No. 2002-41025    Patent Document 2: Japanese Patent No. 3128429